The present invention generally relates to liquid separation components, systems and methods. More particularly, the present invention relates to a liquid flotation separation system, which occupies a much smaller footprint and can be adjusted to accommodate the changing liquid stream.
It is often necessary to remove contaminants from liquid. For example, the need to remove particles, colloids, solvent and oil from wastewater is desirable in many settings.
Most wastewater solid and emulsified components such as soil particles, fats, oils and grease are charged. Wastewater processing/treatment chemicals or additives such as coagulants and flocculents are added to neutralize this charge and initiate nucleation and growth of larger colloidal and suspended particles, also referred to as floccs. Floccs can arrange in size from a millimeter to centimeters in diameter when coagulation and flocculation processes are optimized. Too much chemical will recharge floccs and result in their break-up and/or permanent destruction as overcharged particles or floccs repel each other and tend to stay apart.
Coagulants are chemicals used to neutralize particle charge such as inorganic salts (e.g. ferric chloride) or polymers (e.g. cationic polyamides). Flocculants are large molecular weight polymers used to collect the smaller coagulated floccs into large stable floccs, facilitating solid/liquid separation. These large molecules are often coiled and have to be uncoiled plus mixed well with the incoming coagulated wastewater stream.
Coagulants are often viscous chemicals, requiring adequate mixing time and energy to mix them homogeneously with the incoming wastewater stream. Similarly, an optimum mixing energy is required for the flocculants to be uncoiled and mixed well with the incoming coagulated wastewater stream. If the polymer strands are wound or “globbed” together, the polymer can only attach a minimal amount of waste particles. If mixing is not optimized, an excessive amount of coagulant or flocculant polymer may be introduced into the contaminated liquid in an attempt to coagulate to the greatest extent possible, thus wasting valuable and expensive coagulant and polymer chemicals. However, if too much mixing energy is applied, irreversible break-up of the floccs and inefficient solid/liquid separation occurs.
Dissolved air flotation (DAF) systems are often used to separate particulate material from liquids, such as wastewater. These systems typically employ the principle that bubbles rising through a liquid attach to and carry away particles suspended in the liquid. As bubbles reach the liquid surface, the attached particles coalesce to form a froth that is collected.
It is preferred that the contaminated liquid and treatment additives form a homogenous mixture such that when the dissolved gas is added and subsequently allowed to coalesce into bubbles, a good majority of the contaminants will be taken into the surface with the bubbles. If the mixture is not homogenous, an unacceptable amount of contaminants will remain in the liquid even after treatment.
In the past, it was believed that vigorous mixing over a prolonged period of time provided optimal mixing. However, the inventors have found that this is not the case. Instead, the inventors have discovered that certain treatment additives are very sensitive to the mixing energy used. Thus, over mixing, as well as under mixing, can have deleterious effects on the additives and may alter their behavior or efficiency. The inventors have also found that mixing time for various treatment additives vary according to the mixing energy used. To effectively use coagulants and flocculants, the inventors have found that mixing time and energy must be matched with pressurization and depressurization energy to create bubbles that are the right size to attach to the floccs and create bubbles that grow into larger bubbles after attaching to the floccs. This ensures the flotation of the flocc clusters out of the water and replacement of much of the entrained water in the flocc cluster with air.
Traditional DAF systems select a fraction of the process exit stream and re-saturate this stream with dissolved gas, typically atmospheric air. This fractional stream is discharged into the lower portion of the flotation tank and the dissolved bubbles rise through the liquid and attach to the contaminant particles in the liquid. The probability of attachment is a function of the number of bubbles formed, the bubble sizes, the collision angle, and the presence of hydrophobic attraction of the bubble to the particle. The tank includes an outlet through which treated liquid passes at a flow rate consistent with the inlet rate of the liquid plus the fraction of discharge circulated for air entrapment.
DAF system processing time and contaminant removal efficiency typically depend on the residence time of the bubbles in the solution and the probability of bubble/particle contact. The residence time, in turn, is affected by bubble size, bubble buoyancy, the depth at which the bubbles are released in the flotation tank, and the amount of turbulence in the liquid. Relatively large footprints are necessary to allow the bubbles sufficient time to rise from the bottom of the tank and reach the liquid surface. As a result, traditional DAF systems employ relatively large and costly tanks having correspondingly large “footprints”.
The very size of such systems increases the period of time between control adjustment and effect. This is because water going by the adjustment point, for example a polymer inlet upstream of the DAF, requires over half an hour, and often over an hour, to reach the outlet of the DAF. Thus, there is a substantial delay (i.e. response time) before the effect of the adjustment can be ascertained so as to inform the next adjustment. Thus, these systems lack real-time or even near real-time control. In the event the processing produces a treated effluent stream that is outside operating requirements, the long response time results in production of many gallons of out-of-specification wastewater.
This is especially true under circumstances in which the DAF unit receives flows from several dissimilar processes. This is a common occurrence. Many times the separate flows make up varying fractions of the total flow entering the DAF unit. Floor drains from a canning floor, for example, may carry a fairly small quantity of drained liquid most of the time and large flows during wash downs. Thus, the character of the composite flow that reaches the DAF can commonly change from one minute to the next. Unless adjustments are made to the DAF process, usually via adjustments of chemical dosages, the contaminant removal efficiency will vary and may degrade below requirements. A need exists for the ability to make real time or near real time adjustments that respond to shifts in the character of the streams to be treated. The large tank size of the typical DAF tank is counter-productive to making these real time adjustments.
In an effort to reduce the tank size for a DAF system, one proposal disclosed in U.S. Pat. No. 4,022,696 employs a rotating carriage and floc scoop. The carriage directs an inlet solution substantially horizontally along a flow path to increase the path length for bubble travel, and correspondingly increasing the residence time. However, the rotating carriage and scoop create turbulence that slows bubble rise. Unfortunately, while the tank size reduction is set forth as an advantage, the problem with performance tied to residence time still remains.
Another proposal, disclosed in U.S. Pat. No. 5,538,631, seeks to address the turbulence problem by incorporating a plurality of spaced apart and vertically arrayed baffles. The baffles include respective vanes angularly disposed to re-direct the flow of liquid from an inlet positioned at the bottom of the tank. Liquid flowing through the tank deflects upwardly as it traverses the vanes, purportedly reducing the extensity and intensity of turbulence generated near the inlet to the tank. While this proposal purports to reduce the turbulence problem relating to bubble residence time, the redirected fluid still appears to affect bubbles rising in other areas of the tank, and influences the residence time of such bubbles. Moreover, the proposal fails to address the basic problem of DAF performance being dependent on the need to accomplish bubble-to-particle-adhesion during bubble rise. This increases the residence time needed to complete separation.
In an effort to overcome the limitations in conventional DAF systems, air-sparged hydrocyclones (ASH) have been proposed as a substitute for DAF systems. One form of air-sparged hydrocyclone is disclosed by Miller in U.S. Pat. No. 4,279,743. The device typically utilizes a combination of centrifugal force and air sparging to remove particles from a fluid stream. The stream is fed under pressure into a cylindrical chamber having an inlet configured to direct the fluid stream into a generally spiral path along a porous wall. The angular momentum of the fluid generates a radially directed centrifugal force related to the fluid velocity and indirectly with the radius of the circular path. The porous wall is contained within a gas plenum having gas pressurized to permeate the porous wall and overcome the opposing centrifugal force acting on the fluid. In operation, the unit receives and discharges the rapidly circulating solution while the air permeates through the porous wall. Air passing through the walls of the porous tube are sheared into the fluid stream by the rapidly moving fluid flow. Micro-bubbles formed from the shearing action combine with the particles or gases in the solution and float them toward the center of the cylinder as a froth in a vortex. The centrally located froth vortex is then captured and exited through a vortex finder disposed at the upper end of the cylinder while the remaining solution exits the bottom of the cylinder.
In operation, however, a substantial portion of the froth tends to become re-entrained in the liquid leaving the bottom of the hydrocyclone instead of exiting the top. In addition, froth exiting the top usually has a substantial fraction of water that must then be subjected to lengthy dewatering for decanting back into the process upstream of the hydrocyclone.
One variation in the general ASH construction, as described in U.S. Pat. Nos. 4,838,434 and 4,997,549, includes employing a froth pedestal at the bottom of the cylinder to assist directing the froth vortex through the vortex finder. Another ASH modification includes replacing the vortex finder and froth pedestal with a fixed splitter disposed at the bottom of the cylinder and having a cylindrical knife edge. The edge is positioned to split the helically flowing solution into components dependent upon the specific gravity of the components. As above, the ASH systems tend to suffer from relatively large amounts of solution typically remaining in the froth, and significant particle concentrations often remaining in the solution. In practice, as the particle size of the contaminant becomes smaller, the resulting vector force of the axial and radial velocity dominates the positioning of the particle in the liquid stream. This reduces the effectiveness of the hydrocyclone separator to the point where the smaller particles become randomly distributed in the solution independent of specific gravity.
Morse, et al. disclose in U.S. Pat. No. 6,106,711 a system using a hydrocyclone that differs from the above by the absence of a froth pedestal and vortex finder and by the fact that both the froth and the liquid exit the hydrocyclone together. In addition, the system relies on a downstream tank with vanes that are slanted from the vertical so as to separate the bubble-particle aggregates from the mass of the liquid stream. Morse, et al. also disclose in U.S. Pat. No. 6,171,488 a system using a hydrocyclone that differs from U.S. Pat. No. 6,106,711 in that the hydrocyclone makes a submerged entry into the downstream tank. Although for both of these patents the assembly is small compared to DAF systems, and so provides for near-real-time control, the assembly is a single unit that requires a sizeable location and is large enough to require special equipment to move. It also cannot accommodate the sequential introduction of more than one additive that must be thoroughly mixed with the stream before the introduction of the next additive. For example, it is desirable to adjust pH before adding polymeric flocculants so that high doses of the latter are avoided. In addition, a higher number of extremely fine bubbles would improve flotation. For these Morse inventions, there are not many variables that can be adjusted to optimize performance, so the system often must be customized at the time of manufacturing to the specific waste stream to be treated.
Current technologies are not satisfactory in their ability to respond fast to a changing wastewater influent. The mixing of chemical additives is often physically destructive. They are often not efficient and generally require a long time, causing the real life systems to be large and take up valuable real estate inside the manufacturing facilities.
Therefore, the prior art has not solved the essential problems of large footprints, process control, modular design, homogenous mixing of contaminants, additives and air, or the flexibility to treat the smallest to the largest flows with off the shelf components, or the ability to tune these components on site. A continuing need exists for a flotation separation system with components that need not be near one another so that space constraints can be accommodated. The need also exists for a method of simply and economically creating large quantities of the optimal size bubble needed at each step of the flocculation and flotation process. The need further exists to be able to easily vary the types and order of additives to minimize doses and interface with downstream additives. An additional need exists for a separation system that reduces the quantity of additives needed per unit volume of liquid to be treated. The need exists to control the number, size, and timing of the bubble's formation creating long-range hydrophobic forces acting between the contaminant particles and bubbles, all of which would increase the effectiveness of the system and reduce the operating cost. The flotation separation system and method of the present invention satisfies these needs and provides other related advantages.